Optimized drive control for inverter-fed ultrasonic motors

An optimized control scheme for the most advanced traveling wave type ultrasonic motor powered by a resonant converter is presented and verified by measurements on a prototype drive. Basing on an averaged drive model, which reflects the slow dynamic behavior of the drive's ultrasonic oscillations by time-varying fundamental Fourier coefficients, a cascaded two-phase vector control scheme is designed. The novel drive control divides into an inner voltage and outer bending wave control compensating couplings and suppressing the beat characteristics. Since an amplitude modulation is applied instead of frequency modulation, the well known pull-out phenomenon is eliminated in general. By means of an on-line frequency adaptation to the stator's resonance the drive's performance is optimized remarkably.